The monitoring of the endogenous rhythm of secretion of melatonin. The data show the concentration of plasma melatonin for each group separately (G1 to G4). Animals were housed in constant darkness and given an i.c.v. injection of either metformin (G1 & G2; open triangle) or AICAR (G3 & G4; open square) or artificial CSF as a control vehicle (closed circle). Injections were performed nine days apart and in a random order for each group (see text and Figure 1 for more details about the order of injection for each group). The arrows depict the time of injection and shaded area corresponds to the period of increased sampling frequency. Note the log-scale y-axis.